Many aircraft pitch axis stabilization systems use a vertical or rate gyroscope to provide signals responsive to the attitude or rate of change of the attitude of the aircraft. These signals are often used in conjunction with altitude error, or derived vertical speed signals, to provide control signals for operating pitch control surface positioning means. It is possible, utilizing such positioning means, to develop systems which will fly an aircraft at constant altitude, or provide comfortable closure on a preselected altitude without significant overshoot or hunting.
The dependence of such systems on gyroscopes greatly increases their cost and reduces their reliability. Often because of size or weight limitations such systems are suitable only for large expensive aircraft and not the general aviation field, where cost and weight are important considerations.
Systems as outlined above have improved to a certain extent, at least in the area of altitude hold systems, with the use of pressure transducers to provide altitude reference signals, and the use of digital hold circuits to store predetermined values of these signals. The versatility of operation of these systems, however, has been limited.
Mechanical accelerometers have occasionally been used to provide signals to aid in pitch axis stabilization. One difficulty associated with such apparatus is the fact that the accelerometer responds to acceleration in directions not associated with vertical motion of the aircraft during certain maneuvers, leading to pitch axis instability. Another difficulty is the great expense generally associated with the installation and use of such accelerometers.